A superabsorbent polymer (SAP) is a synthetic polymeric material capable of absorbing moisture of from 500 to 1000 times its own weight. Various manufacturers have called it by different names, such as SAM (Super Absorbency Material), AGM (Absorbent Gel Material), etc. Since such superabsorbent polymers started to be practically applied in sanitary products, they are now being widely used not only for hygiene products such as disposable diapers for children, etc., but also for water retaining soil products for gardening, water stop materials for civil engineering and construction, sheets for raising seedlings, fresh-keeping agents for food distribution fields, materials for poultices, and the like.
As a preparation process for such superabsorbent polymers, a process by reverse phase suspension polymerization or a process by solution polymerization has been known. For example, Japanese Patent Laid-open Publication Nos. S56-161408, S57-158209, and S57-198714 disclose the reverse phase suspension polymerization. In particular, a large amount of fine powder is generated during the preparation process of such superabsorbent polymers. Because all the fine powder cannot be discarded, the fine powder is mixed with a base resin of the superabsorbent polymer, and the mixture is reassembled and utilized as a product. However, it is better for a smaller amount of fine powder to be generated. If single particles of the superabsorbent polymer have lower strength, it is highly likely that the single particles become fine powder during the preparation process. Thus, many attempts have been made to increase strength of the particles, but there was no way to quantitatively and reproducibly evaluate whether or not the strength is increased after the attempts. Particularly, according to a known method of measuring a generation amount of fine powder from the superabsorbent polymer, a predetermined amount of the superabsorbent polymer is pulverized, and then the amount of fine powder is directly measured. Thus, there is a problem that it is difficult to quantitatively and reproducibly evaluate the fracture strength of single particles themselves of the superabsorbent polymer.
Accordingly, it is necessary to develop a method of effectively measuring strength of single particles of the superabsorbent polymer in order to predict the amount of fine powder which may be generated during the preparation process of the superabsorbent polymer.